1. Field of the Invention
The invention relates to a control method and to circuit arrangements to operate a high intensity discharge lamp with a lower frequency rectangular current waveform generated electronically by a higher frequency inverter.
2. Description of Background Information
In electronic high intensity discharge lamp ballasts, there are two distinctly different methods to drive the lamp. The first method is to drive the lamp with high frequency sinusoidal current, and the second is to drive the lamp with low frequency rectangular current. Many efforts have been made to stabilize the HID lamp operation with both the high frequency sinusoidal and the low frequency rectangular driving method.
U.S. Pat. No. 4,373,146 to Robert R. Bonazoli et al., issued Feb. 8, 1983, discloses a method of operating HID lamps in which frequency modulation of a carrier waveform in the kilohertz range is used to provide a variable high frequency AC output. The variable high frequency AC output is then applied across the HID lamp to operate the lamp in a manner that minimizes or avoids acoustic resonance. As further discussed in the above-noted patent, the arc instability caused by acoustic resonance depends upon both the shape of the carrier waveform and the shape of the modulating signal. The patent discloses that a rectangular carrier waveform is much more desirable than a sinusoidal carrier for arc stability, and that a saw tooth modulating signal of between 1 ms to 10 ms per cycle with a retrace time of less than 1 .mu.s is better than a triangular modulating signal. One drawback of the technique disclosed in this patent is that the lamp is still driven by a high frequency current in the kilohertz range. Due to the complexity of the acoustic resonance, arc instability may occur with lamps made by different manufacturers, different batches of the same type of lamp, and lamps at different points in their service life (new, seasoned, and end of life). It is very difficult, if not impossible, to evaluate all the available lamps for acoustic resonance, and moreover, to evaluate the available lamps for acoustic resonance at different times in their service life. Another drawback of this disclosure of this patent, which is not mentioned but is implied by the disclosure, is that the acoustic resonant frequency of a lamp should be avoided at near the lowest operating frequency or near the highest operating frequency. If the acoustic resonant frequency of a lamp is either near the lowest operating frequency or near the highest operating frequency, arc instability may occur. This phenomenon can be explained using frequency domain analysis. In the frequency domain, it can be found that the magnitude of harmonics is highest on two edges of the frequency spectrum, which corresponds to the frequencies near the lowest or the highest frequency in the time domain.
U.S. Pat. No. 5,569,984 to Antonius H. Holtslag, issued Oct. 29, 1996, describes another method to drive a HID lamp with high frequency sinusoidal current. The complicated control circuitry disclosed in this patent constantly detects the conductivity of the HID lamp and the operating frequency. Selection of the operating frequency to avoid acoustic resonance is based on the evaluation of the deviation of the lamp conductivity. After the ballast finds the lowest deviation of the lamp conductivity, the operating frequency is then temporarily fixed until the deviation exceeds the predefined limits. The intelligence of the disclosed control scheme lends itself for driving HID lamps with high frequency current under different conditions, such as aging of the lamp, different manufacturers, different types of HID lamps, and different batches of the same type, over a certain operating frequency range. A significant drawback of this disclosure is the complexity of the control circuitry. Another drawback of the disclosure of the above-noted patent is that the algorithm for differentiating true acoustic resonance from any other arc instability (such as arc jumps, flare ups, and arc movement caused by any mechanical movement of the lamp) may actually need to be more complicated than the double scan algorithm disclosed.
A paper titled "White-Noise Modulation of High-Frequency High-Intensity Discharge Lamp Ballasts" by Laszlo Laskai, in IEEE Industrial Applications Society Conference, 1994 (IEEE Pub. 0-7803-1993-1/94), and a Ph.D. dissertation (Texas A&M University, College Park, Tex.) entitled "High-frequency ballasting techniques for high-intensity discharge lamps" by the same author discuss high frequency sinusoidal current operation with white-noise modulation to avoid arc instability. It achieved better results than the frequency modulation (FM) method. However, the white-noise method has significant drawbacks as well, in that an operating frequency range having a portion free from acoustic resonance must be found. Otherwise, the arc will not be stable, primarily because of acoustic resonance.
Driving high intensity discharge lamps via low frequency rectangular current driving, in general, is a better method than the high frequency (sinusoidal or rectangular) waveform. The industry, by virtue of experience, accepts that to avoid arc instability due to acoustic resonance, the ratio of a superimposed high frequency switching ripple current to the low frequency driving current has to be sufficiently low, usually below 10%.
U.S. Pat. No. 4,904,907 to Joseph M. Allison et al., issued Feb. 27, 1990, discloses a modified buck topology in which an LC parallel resonant network is inserted into the buck inductor. The inserted LC parallel resonant network has its resonant frequency at the buck operating frequency. The ripple current of the fundamental switching frequency of the buck power regulator is attenuated significantly by the resonant network, and the high frequency ripple current through the lamp is much less than the low frequency rectangular lamp current. Arc instability due to acoustic resonance will not occur. However, this method has its own drawbacks, in that the attenuation factor is highly sensitive to the frequency variation of the buck converter. A small decrease or increase in switching frequency will adversely affect the high frequency ripple current to low frequency lamp current ratio. If the ripple exceeds the threshold of acoustic resonance, arc instability may occur.
In low frequency electronic high intensity discharge lamp ballasts disclosed in the prior art, the higher frequency ripple superimposed on the lower frequency rectangular current has to be attenuated below a certain threshold (usually below 10%) to avoid acoustic resonance. One method to attenuate higher frequency ripple is to have large capacitance and small inductance in the LC low-pass output filter network. The inductor is in discontinuous current mode and the switching elements are in zero current switching, and the efficiency of the switching elements is therefore high. However, the physical size and the cost of the capacitor increases. Another method to attenuate higher frequency ripple is to increase the inductance and the capacitance of the LC low-pass filter. The inductor is now in continuous current mode and the switching elements are in hard switching mode. However, for this method, the efficiency is low due to increased switching losses, and if resonant ignition is used, the problem is further complicated. The discontinuous current mode with large capacitance cannot be used for resonant ignition due to extremely high circulating current. To use resonant ignition while maintaining low ripple, the capacitance needs to be small and the inductance needs to be large.